Electrical apparatus



June 26, 1951 s. BREEN ET AL ELECTRICAL APPARATUS 2 Sheets-Sheet 1 FiledApril 3, 1950 HLSW@ wai/KM QM@ d June 26, 1951 Q BREEN ET AL 2,558,670

ELECTRICAL APPARATUS Filed April 5, 1950 2 Sheets-Sheet 2 A In vena/".5

JaaW/ef/ reei? zxzae/ L. Ker/f @@Mslww Patented `lune 26, 154951ELECTRICAL APPARATUSA Stanley Breen, Geneva, and Maxwell A. Kerr,

Wheaton, Ill., assignors to Operadio Manufacturing Co., a corporation ofIllinois Application April 3, 1950, Serial No. 153,552

20 Claims. (Cl. 175-320) This invention relates to an electricalapparatus and particularly to a sensitive relay. The sensitive relay towhich this invention pertains is adapted to work with sources orgenerators having an extremely low potential direct current output.

Generators of the type having a Search coil operating in the earthsfield, as one example, provide a direct or slowly varying current ofminute value at low potential. Such currents in extreme cases may bemeasured in microamperes with potentials in microvolts. In connectionwith such devices, it is desirable to provide a relay which will respondto the output of such a generator and close a second relay circuit forsupplying current to a final load.

Vacuum tube amplifiers for such purpose are not satisfactory. One reasonis that a conventional vacuum tube amplifier is inherently unsuitablefor operation on slowly varying or direct currents. Even elaborate meansto balance the amplier and prevent drift still make the operation ofsuch amplifiers erratic and unreliable. Secondly, is the fact that avacuum tube amplifier requires a cathode circuit continuously energized.In certain types of apparatus, such as portable apparatus for neld useor apparatus having long stand-by periods, the matter of a power supplyfor energizing a cathode circuit is a serious problem and renders a,vacuum tube amplifier imr part of a relay circuit. Since moving coilgalvanometers are sensitive and have very Weak forces normally generatedtherein, it follows that the movable element is unlikely to make goodcontact between the xed and movable contacts.

The most sensitive type of galvanometer is the moving coil type havingcurrent leads and torsion suspension combined into one structure. Themovable contact will naturally move with the coil and may have aseparate lead therefor or use the moving coil leads. two leads for themovable part of the galvanometer renders the structure impractical andimpairs the sensitivity. If two leads are used in a conventional manner,a serious drawback becomes apparent. When the relay circuit is estab-Having more than f lished, the IR drop across the galvanometer coilsystem, including suspension, becomes large in comparison tc the E. M.F. normally developed by the original generator as a search coil. Hencethe entire movable coil system becomes a source of E. M. F. which mayexceed the safe value to be impressed across the original generator as asearch coil with disastrous results to the latter.

This invention provides a construction utilizing a movable coil type ofgalvanometer whereby the movable element, when it reaches apredetermined position, is adapted to lock itself closed so that rmcontact is established between the xed and movable relay contacts. Theinvention further provides means whereby relay currents are preventedfrom flowing into the generator, thus removing the possibility of damageto the generator due to excessively large currents. This inventionfurther provides means whereby the coil will unlock itself when desiredand open the relay circuit in preparation for a new relay operatingcycle. A galvanometer embodying the invention utilizes the fullsensitivity of the instrument and, apart from the movable contact arm,does not have elements upon the movable coil which detract eitherelectrically or mechanically from the sensitivity of the galvanometer.

The invention, in general, is characterized by a combination of a novelgalvanometer structure and a novel circuit for the galvanometer. Thegalvanometer is of the moving coil type operating in a constant magneticfield. The movable coil is preferably, though not necessarily, suspendedin a torsion type of mounting. The mounting also functions to leadcurrents to the movable coil. The movable coil carries at least oneInovable contact in the form of a thin wire extending therefrom. Thiscontact is connected to the movable coil at any place desired betweenthe end and center of the coil. Two movable contacts may be thus carriedand be connected symmetrically on opposite sides of the center point ofthe movable coil. While the movable contact may be connected to theterminal of the movable coil, it is preferred to make the connection toan intermediate eccentric point on the coil proper. The amount ofeccentricity of the contact tap on the coil will be determined by otherconsiderations which will be apparent in connection with a detaileddescription of the invention.

The eccentricity of the contact tap on the movable coil is withreference to the electromagnetic properties of the coil. Thus the centerof the movable coil will be that point which will divide the coilelectromagnetically into equal and opposite coil parts. While such acoil center is near the geometrical wire center, it is not necessarilycoincident, due to departure from theoretically perfect winding. Hencethe term coil center will be used here with reference to theelectromagnetic eiects and not the geometrical center or resistancecenter unless expressly stated.

An important feature oi the invention resides in the provision forconstructing a winding so that an eccentric tap i'cr a movable contactmay be substantially at the resistance center of the winding so that thedirect current resistance of the' parts of the complete winding onopposite sides ci the tap are substantially equal. This may be true eventhough two movable contacts are pro-- vided. This effect is obtained byforming that part of the movable coil at the electrical or resistancecenter for equal distances on both sides of the center up to the movablecontact tap or taps of wire having lower resistance than the remainderof the movable coil. In so disposing the low resistance section of themovable coil, it is assumed that the geometric center and ohmicresistance or electrical center of the coil are coincident. In practice,these two points are close enough so that satisfactory results areobtained.

By virtue of the above expedient, the galvanometer movable contact (orcontacts, if two are used) is electromagnetically eccentric of thewinding but electrically is substantially centered. Hence thegalvanometer coil is substantially symmetrica] for ohmic resistance asseen from the movable contact or contacts and coil terminals.

rEhe relay circuit in which the galvanometer operates has a source ofconstant potential, such as a battery, and a three wire type of load. Byvirtue oi the arrangement of the relay circuit source of potential andthree wire load in the relay circuit and the inclusion in the circuit ofsuitable balancing means, it is possible to provide a substantiallyperfect electrical balance across the terminals ci the galvanomcter whenthe load relay circuit is closed. As previously pointed out, the movablecoil may be balanced for IR drop. The torsion suspension leads, however,may vary from instrument to instrument so that resistance balancingmeans external to the galvanometer are preferable. When balance at thegalvanorneter terminals is obtained substantially no current from theload relay circuit will flow through the generator.

Where two fixed relay contacts are provided, it is possible to utilize acenter Zero type of nstrument, thus rendering the instrument independentof polarity of the generator. Normally, the generator must be connectedin such a manner that current of proper polarity actuates thegalvanoineter coil in the direction so that the relay contacts approacheach other.

In order that the invention may be understood, it will now be explainedin connection with the drawings wherein Figure 1 is a diagrammatic viewshowing one form of the invention and Figures 2 to ll inclusive arediagrammatic views showing modified forms of the invention.

Referring first to Figure l, generator lll of any desired type is shown,this generator having terminals l l and l2 and furnishing, at desiredtimes or under predetermined conditions, a current whose intensity isnormally measured in microamperes at a potential as low as severalmicrovolts. Such a generator, for example, may consist of coil ll ofwire with or without a suitable ferromagnetic core and adapted tofunction as d an exploring coil. Such a generator will have an internalresistance represented by resistor I5. rlhis generator, in addition, mayalso have inductance and capacitance, these two properties of thegenerator' being unimportant for the purposes at hand.

Terminals ll and i2 of the generator are connected by wires ll and li?to terminals 20 and 2| oi a galvanometer generally indicated by numeral22. Galvanometer is preferably of the suspension type although theconventional spiral hair spring Dnsonval 'type of galvanometer may b'eused. inasmuch as both types or galvanorneters are well known and highlydeveloped, a detailed description of the galvanometer itself is notdeemed to be necessary.

Galvanometer 22 has suitable permanent magnet system it having poles 2tand 25 respectively. In practice, the magnet would have suitable polepieces ci soit iron shaped in a particular manner to distribute the fluxfrom the magnet over a predetermined angular extent, all well known inthe art. Cooperating with the magnet is movable coll winding which mayconsist of any desired number of turns on a suitable form. Coil 2.8 hasterminals Erl and til attached to suspension wires tl and 32, thesewires being aligned and consisting or ilegible, elastic good conductingmaterial such as gold, tungsten, copper, bronze or any other material.Wires 3i and 32 may be tensioned by suitable means not shown, in amanner well known in the art with suitable adjusting means so thatmoving coil normally has a predetermined zero position.

Between one oi the galvanometer terminals and the suspension wire, inthis case between terminal 2l and wire t2, there is provided electricalbalancing means consisting or' variable resistor and variable inductcrshunted by Variable resistor 35 and variable capacitor 36 for a purposewhich will be apparent later. In some instances, it may be possible toeliminate either or both resistors cr the inductor or combine the two inone unit. in the case of a DArsonval type of meter, the conventionalspiral springs would replace the suspension wires.

Movable coil carries movable relay contact El. This contact may consistof a ne wire eX- tending outwardly from the coil s-o 'that this wiremoves over a predetermined path when coil 28 moves in response to acurrent passing through the same. Movable contact 55? is connected topoint 28h on coil somewhat off electromagnetic center 28a of the lecilso that coil M3 is divided into two portions. In practice, coil 2t mayconsist of a nrmber ci turns cl wire, as 25 turns for example. Between28a and 2gb there may be part oi a turn or a number or turns.

Coil E3 normally 'would of ine copper wire such as #au an example.However, between points 28h and ttc on the winding, these points beingsymmetrical to point it is preferred to use wire having substantiallyless resistance per unit length than is true of the winding as a whole.Thus if coil is or #4G wire, thenwire 25h-#Btc may cf #34; or #86 wire.

If #Si wire is used, the resistance will be 1/8' that of the wire, thewire ses n doubling for each two steps wire gauge. The low resistancepart ESD--Zlc may of silver to further reduce the resistance, althoughthe gain is ordinarily not worth the expense.

The departure of point from electrical center 28a. of coil il' dependsin some measure upon the amount oi relay current Flowing in the relaycircuit when closed, the amount of current which the galvanometerwinding can safely carry, the amount of force necessary for locking andunlocking the relay contacts and the convenience of locating a suitablespot on the winding to which a connection be made.

Cooperating with movable relay contact El is fixed relay contactconnected by wire 6l to terminal t2 of double pole double throwreversing switch 3.1. This switch has terminals 43 and 44 going tosuitable movable contacts. The movable contacts cooperate with fixedcontacts 42 and 45 at one switch position and contacts 42' and 45 at theother position or" the switch. Movable contact terminals 43 and Ml areconnected to the terminals of a source of potential for the relaycircuit, this being conveniently shown as battery terminal 44 beingconnected to the positive terminal and terminal 43 being connected tothe negative terminal, as an example. The polarity ci the connections,as will be readily apparent, is unimportant and is given merely by wayof illustration.

Terminal 45 of the switch is connected by wire 53 to intermediateterminal Si of a three wire load. This load, as shown here, consists ofwindings 52 and 53 having common intermediate terminal 5l and outerterminals 5c and 55 respectively. Terminal 5e is connected by wire 5E toterminal 2G while terr .na is connected by wire 5l to terminal Windings52 and 53 are the windings of load relay these windings being soarranged that when current from terminal El goes through windings 52 and53 as shown, the magnetic elds created by the two windings will aid eachother. Load relay 58 is a conventional relay and has contacts and El? ofusual construction going to a utilization circuit of any typewhatsoever, contacts 59 and 60 being adapted to handle comparativelyheavy currents.

The resistances of windings 52 and e3 of relay 58 are preferablysubstantially greater than the resistance of winding 2li. This has theeffect of preventing excessive loading of generator IQ.

The operation of the relay system is as follows: Assume that double poledouble throw switch Se is in the position shown and the galvanometercoil is at rest. Now assume that generator lil becomes active andgenerates current with terminal l! being positive and terminal l2 beingnegative. Current will flow from terminal l! along wire il to terminal2,53 through the moving coil portion ci the galvanometer to terminal 2land back along wire i8 to terminal l2 oi the generator. Movable coil 28of the galvanometer begins to respond to this current, it beingunderstood that the polarity of the current is such that movable contact31 approaches Contact eil. Assume that movable contact 3l reaches ixedcontact 4e. Even though contacts 3l and il do not close tightlynevertheless, some current from relay battery 43 will flow. A circuitfor this current may be traced as follows: From the negative terminal ofbattery t to terminal 43, terminal 42, relay contact llt, relay contactEl, tap 28h of coil 28, terminal wire 32. resistor 33 and inductor 34 toterminal 2i wire Si?, through winding 53 of the load relay, centerterminal 5I, wire 5G to terminal [i5 and then to the positive terminalof the battery. An additional circuit through which relay current willflow er;-

tends from the negative terminal of battery 48 through to movablecontact 3l of the galvanometer, then tap 23h of coil 28 and up toterminal 20 of the galvanometer and then through winding 52 of the loadrelay and then back to the positive terminal of the relay circuitbattery.

Whether this initial current is strong enough to operate load relay Eilis unimportant. The initial current from the relay battery passing alongwire 56 is of the same polarity as current from generator le as far asits eiect upon that part of the coil between 28h and 29 is concerned.The current going along wire 5l to terminal 353 and tap 23h opposes thegenerator current in the galvanometer coil. Thus the eccentricity ofconnection of movable contact 3l causes the currents from the load relaybattery to have unequal and opposing eiects upon the galvanometer coil.Thus as shown, current from wire 5e passes through more than one-half ofcoil 28 and since it aids the current from generator l, it will be notedthat coil 2S will tend to close contacts 3l and A@ tightly against eachother. As soon as suicient current passes through the two contacts inthe galvanometer, load relay 5S will be operated to close its contacts59 and El).

It will be evident that the galvanometer contacts will remain in lockedposition now independently of the condition of generator lil. Now assumethat it is desired to open the galvanometer contacts and restore thesystem to the position shown. To do this, it is necessary to operatedouble pole double throw switch 39 so that terminals 43 and le areconnected to terminals 45 and 42' respectively. It will be observed thatthis reverses the polarity of battery d8 with respect to the coil of thegalvanometer and load relay 58. As far as load relay 58 is concerned,the direction of current is unimportant but it is obvious that thedirection of current flowing through coil 28 is important. Thus, if thenet effect of currents flowing through galvanometer coil 28 is reversed,coil 28 will tend to move contact 3l away from xed contact 4e. Hence,sufcient force can be generated to open these contacts even though somesticking may occur. Thereafter, the reversing switch may be returned toits original condition and the galvanometer is ready to operate anew.

It will be observed that any difference in potential between terminalsZ5 and 2l will result in current ilow through generator l. It isdesirable to limit such potential diirerences as may exist to the orderof that developed across the generator by virtue of generator operation.Durn ing steady state conditions, existing while contacts 3'! and 4G areopen or closed, a high degree of balance across terminals 2o and 2l maybe readily attained. This is true even if part 28h-28o of the coil isnot of lower resistance than the remainder of the coil. Thus anydifference in resistance between tap 22h and terminal 2li on the onehand and tap 28h and. terminal 2l on the other hand, solely through thewinding and suspension, can be compensated for by resistor 33. Hence,after steady state conditions have been reached with the galvanometercontacts closed, the potential difference across 2&3 and 2l may be madezero or small enough so that generator lll will not be damaged.

' It is clear that transients will be generated in the galvanometersystem when the galvanometer contacts open or close. An important causeof the transients is believed to reside in the mutual inductance betweenthe parts of the movable coil considered with reference to a tap whosecontact is just closing or opening. Thus if contact 31 is just closingagainst fixed contact fill, the entire movable coil may be considered asconsisting of one part from terminal 29 to tap 28h and the other partfrom tap 2th to terminal Sii. Obviously, the two parts of the entirecoil are electromagnetically coupled and have mutual inductance. Uponinitial closure of the contacts, the mutual inductance has some effectupon the coil tending to move the contacts together more closely. At thesame time, the transient or transients generated in the coil result in apotential difference at the galvanometer terminals apart from theresistance drop in the coil. This transient may cause a surge ingenerator coil M. By choosing proper values for resistor 35, capacitor35 as weil as inductor 36, a low pass lter may be provided to suppresstransients. A condenser across the generator winding may also help.

When the reversing switch is opened, the mutual inductance effects havesome tendency to generate forces to move the coil in a direction to openthe contacts. While this may be relied upon in some instances, it ispreferred to move the switch to the reversing position. The cornbinationof transient effects and positive actua tion of the movable coil sufficeto move the coil quickly into contact opening position.

The transient effects resulting from opening the reversing switch aregenerally more powerful than from contact closure. Thus after contactclosure, opening of the reversing switch will cause the current to decayat a sharper rate than is true of the current increase at contactclosure. However, the rates oi current rise and decay may be assumed tobe of the same order so that a low pass lter will generally be effectiveagainst all transients.

It is clear from the foregoing, that the polarity of generator lo isimportant when connecting the generator to the galvanorneter system. Forcertain purposes, it may be desirable to have a system wherein thepolarity or" the generator is not important. This is shown in Figurewherein similar' parts carry similar numbers. n this modification,movable contact 3l is connected as before to tap 2573 of coil 28. A lowresistance conductor is between points 23o and 28e. In the system shownin Figure 2, the suspension system for movable coil 28 is adjusted sothat the normal aero position of the winding is the same as in a aerocenter type of instrument. In such an instrument, current in onedirection will cause the coil to move one way while current in reversedirection will cause the coil to move the other way.

Cooperating with movable contact Eil are fixed contacts fil! and ldd onopposite sides, these two contacts beingarranged so that the movablecontact can close against one or the other, de* pending upon thedirection of coil movement. Fixed Contact do is connected to contact i2of double pole double throw reversing switch 39 exactly as in Figure l.Fixed contact Elia is connected by wire flic to the correspondingContact c2c of double pole double throw reversing switch Bca. The tworeversing switches are the same and are connected in the same manner,the corresponding part carrying the letter a with the exception thatbattery d8a has its positive terminal connected to contact 53o whereasin the lower reversing switch, Contact (i3 is connected to the negativeterminal of this reversing switch.

8 As clearly indicated, the two reversing switches are tied to besimultaneously operated.

The operation of the modified system is as follows: If the polarity ofterminal H of the generator is positive as in Figure l, then the actionof the system is exactly the same as in Figure l, coil 28 movingclockwise as seen in Figure 2 to close a circuit with contact dii. lf,however, the polarity of generator it is reversed, coil 28 will moveanti-clockwise and Contact 3'! will approach contact 5%. When contacts3l and Mia touch, current from battery lac will act in the same manneras from battery iii in Figure 1, the polarity of 4ta being such thatcoil 23 is driven to close contact 3l against contact ma more tightly.

The operation oi the system as far as the load or secondary relay isconcerned, is the same as in Figure 1 and needs no explanation. In orderto unlock the contacts with the system shown in Figure 2, it will benecessary to throw the two reversing switches from the position duringrelay operation, such as shown in Figure 2, for example, to the reverseposition.

Referring now to Figure 3, a further modification is shown. Thegalvanometer is generally the same as in Figure 2, having one movablecontact operating between fixed contacts. However, terminals 2li and 2lof the galvanometer are connected directly to the coil suspension and donot have any resistors, capacitors or inductors as in lead 32 ofFigure 1. instead, generator coil Ill has condenser ida across it and isin series with inductor lftb across galvanometer terminals 2c and 2 lTerminals 2o and 2l are connected to resistors 82 and 83 having commonjunction 8d. Wire 5o is connected through conventional relay 35 tojunction M. The galvanonieter load here is still of the three wire type,consisting of resistors 32 and 33 and relay E5. Resistors 82 and 83 arenot necessarily equal and may be proportioned relatively the saine asthe resistances from tap 23h to terminals Zii and 2i.

This system may be considered as a bridge with galvanometer partsZlib-dii as one arni, 23h-2 l as a second arm, resistor 32 as a thirdarrn and resistor 83 as a fourth arm. The diagonal across 2t and 2i isformed by the generator and inductor in series. A. difference inpotential is impressed across the second diagonal from movable contact3;? and junction il@ when the galvanometer contacts are closed. Forperfect operation, the bridge arms should be so related that the bridgeis substantially balanced across 2tand 2l'. As has been previouslypointed out, this balance may be substantially achieved for steady stateconditions by choosing proper values for resistors 82 and 33. By properdesign of the rst diagonal containing the generator and inductor, a lowpass filter effect is provided.

It will be noted that the system illustrated in Figure 3 resembles thesystem of Figure 2 in re-l duiring two sets of batteries in thegalvanoineter relay circuit. This be undesirable. Reerring now to Figurefi, a modification is shown wherein only one battery necessary.

This system has generator Mc connected in series with resistor lea(which resistor may be the resistance of Hic) across galvanorneter terminals 20a rand 2id. These terminals are connected directly tosuspension elements 'dic and 32a going to terminals 29a and sila, ofcoil 2S. Coil 28 is the same as in Figure 1 except that it carries twomovable contacts Elib and 31o at points 2817 and 28e respectively. Thepart of the coil between these contact taps preferably has a lowerresistance than the remainder of the coil. Contacts 31h and S'lccooperate with stationary contact 42a. Contact Mia is connectedtocontact 42a of a double pole double throw switch having contacts 42a,c2c. 45a, 45a. movable contacts 43a and Mic. This switch is connected asin Figure l so that battery 53a may have its polarity reversed. Fromcontact 55a, wire 50a goes to relay 85a, the other terminal of thisrelay going to junction 84a, or" resistors 32a and i3c. Resistors 82aand 83a are connected to galvanometer terminals 20a and 21a.

It is clear that the galvanometer is the center zero type. Coil 28 willmove irrespective of the polarity of the generator. Normally the oneixed contact doc will be between the two movable contacts. Whichever waythe coil will move, contact closure will result in locking action. Bysimply opening the double pole double throw switch, contact opening mayresult due to the action of the mutual inductance of the coil parts, aspreviously described. However, throwing the switch to the other positionto reverse the battery will be found generally preferable and morereliable.

In general, where extreme sensitivity is desired, the galvanometercontacts must be ne and threaded to remain clean. A rugged but sensitivegalvanom-eter construction is possible7 utilizing various constructionaldetails well known in the Under certain conditions, the entiregalvanometer be immersed in a liquid, as kerosene so that the contactswill remain clean for periods of time.

The load must be of the three wire type and the various circuit branchesof the load preferably should have the same symmetry when coupled to thegalvanometer system and generator would be required by an electricalbridge. Where a galvanom-eter coil carries two movable contacts, as inFigure 4, the two contacts may be considered as one for bridge analysis.Thus one arm of the bridge may consist of one coil suspension and thecoil part going to the nearest movable contact Slo. The other arm maycon- .sist of the other coil part going to the other movable contact,lill). The part of the coil between contacts 31h and 31o may beconsidered as a connection. having negligible effects on the bridge.actually, precise analysis would demand that whichever movable contactis closed against the iixed contact, is the bridge point, thetemporarily idle movable contact being disregarded.

What is claimed is:

l.. A sensitive relay comprising a movable coil type of galvanometer,the movable coil consisting' of a `winding of wire having end terminals,a Contact movable with said coil and connected to a point on said coilelectromagneticallv offcenter, a iiXed contact cooperating with saidmovable contact and touched by said movable contact when said coil hasresponded a predetermined amount to energization, a three wire loadhaving an intermediate and end terminals, a connection from one terminalof the movable coil to one end terminal of the load, a connection fromthe other terminal or" the movable coil to the other end terminal of theload, a connection including at least one source of potential betweenthe ixed contact and intermediate terminal of the load, a source ofminute substantially direct current connected to said coil terminals,said sources of potential and current being properly poled whereby whensaid movable coil responds to current and closes the movable contactagainst the iixed contact, a relay circuit is closed With currentsflowing between the movable contact and terminals of the movable coil inopposite directions generating a net electromagnetic coil effect due tothe eccentricity of the connection of the movable contact to the movablecoil tending to move the coil into tighter contact closing position.

2. The systeml according to claim l wherein only one source of potentialis provided, this source being disposed in the circuit branch having theintermediate terminal of the load.

3. The system according to claim 1 wherein only one source of potentialis provided in the circuit branch having the intermediate terminal ofthe load and wherein switching means are provided for reversing thepolarity of this source of direct potential, said reversal serving togenerate a force in the coil for opening the contacts.

4. The system according to claim l wherein one of said coil connectionsincludes impedance balancing means whereby upon closure of the loadcircuit, no difference of potential due to the source of potential willbe present at the terminals of the minute current source when connectedthereto.

5. The system according to claim 1 in combination with a source ofminute potential.

6. rlhe system according to claim 1 wherein said galvanometer coil has asection symmetrically disposed with respect to the electrical coilcenter and extending to the movable contact tap, said section havingsubstantially less resistivity than the remainder of the coil.

7. A sensitive relay system.l comprising a galvanometer of the movablecoil type, said galvanometer having a movable coil with terminals, acontact movable with said coil and connected to a point on said coilelectromagnetically offcenter, said galvanometer being of the Zerocenter type, a pair of fixed contacts, said contacts being on oppositesides of the movable contact and spaced so that said movable contactmust move from a zero position in either direction for a predeterminedamount of travel to encounter one or the other of said Xed contacts, athree wire load having outer terminals and an intermediate terminal, asource of potential connected between the load intermediate terminal andone fixed contact, a second source of potential connected between theload intermediate terminal and the other fixed contact, the polarity ofthe fixed contacts being opposite to each other, and connections betweenthe outer terminals of the load and the terminals oi the movable coil,said movable coil terminals functioning as terminals for connecting anenergizing source of minute current to said system to move said coilfrom its center Zero position in one direction or the other dependingupon the polarity of the minute current source, said movable contactwhen touching a Xed contact completing a load circuit which tends tolock the contacts, the polarity of the potential sources being such thatwhen the movable contact touches a iixed contact, the load currentthrough the movable coil generates a resultant force to lock thecontacts together.

8. The system according to claim 7 wherein reversing means are providedfor both sources of potential to reverse the polarities wherebyunlocking of the contacts is provided.

9. The system according to claim 7 wherein impedance balancing means areconnected in the movable coil circuit so that no relay potenc cil tialappears. across the minute current source when connected.

1 0. The system according to claim 'l wherein the branch of the systemcontaining the minute current source includes low pass filter means forsuppressing transient effects.

11. The system according to claim 'l wherein said galvanometer coil hasa section symmetrically disposed with respect to the electrical coilcenter and extending to` the movable contact tap, said section havingsubstantially less resistivity than the remainder of thev coil.

12. A sensitive relay system comprising a galvanometer of the movablecoil type, said galvanometer having a movable coil with terminals, apair of contacts` movable with said coil and connected to points on saidcoil electromagnetically oi center butY generally symmetrically withrespect to the electrical center of the coil, said galvanometer being ofthe Zero center type, a xed contact, said movable contacts being onopposite sides of the fixed contact and spaced so that' said movablecontacts. must move from a zero position in either direction for apredetermined amount of travel to encounter the xed contact, a threewire load having outer terminals and an intermediate terminal, a sourceof potential connected between the load intermediate terminal and saidfixed contact, connections between the outer terminals of the load andthe terminals of the movable coil, said movable coil terminalsfunctioning as terminals ior connecting an energizing source of minutecurrent to said system to. move said coil from its center zero positionin one direction or the other depending upon the polarity of the minutecurrent source, one of said movable contacts, when touching the XedContact, completing the load circuit, the polarity of the potentialsource being such that when the load circuit is completed, currentthrough the movable coil generates a resultant force to lock thecontacts together.

13. The system according to claim 12 wherein switch means are providedfor reversing the polarity of the potential source so that after contacthas been established, reversal of the potential source will result in aforce being generated in the coil tending to unlock the contacts` andmove the coil away from contact making position.

14. The system according to claim 12 wherein said load consists of apair of resistors connected in series across the movable coil andwherein the junction of the resistors is connected to the fixed contactthrough a load relay.

15. A sensitive relay galvanometer for use with a three wire type ofload, said galvanometer being of the movable coil type, said coil havingend terminals, at least one contact carried by said coil and movabletherewith, a connection between said contact and a point on said coilelectromagnetically off-center from said coil, at least one fixedContact cooperating with said movable contact, said galvanometer havingits coil terminals for connection to a source of current for moving saidcoil toward a contactmaking position, said galvanometer also having itscoil terminals and xed contact for connection to the end andintermediate terminals re: spectively of a three wire type load with asource of potential included in the intermediate branch of the load,with the potential source of polarity i2 being so` selected that uponcontact closure due to: galvanometer coil movement, a force is generatedin the galvanometer coil tending to increase the force closing thecontacts.

16. The galvanometer according to claim 15 wherein the galvanometer coilhas a section symmetrically disposed with respect to the electrical coilcenter and extending to the movable contact tap, said section havingsubstantially less resistivity than the remainder of the coil.

17. The galvanometer according to claim 15 wherein said galvanometercoil carries two movable contacts, said two contacts being connected totwo points on said coil symmetricallyv disposed with respect to the coilcenter, said galvanometer being of the zero center type with one xedcontact.

18. The galvanometer according to claim 17, wherein said galvanometercoil has a section symmetrically disposed with respect to the electricalcoil center and extending to the points of connection of the movablecontacts, said section having substantially less resistivity thanV theremainder of the coil.

19. A galvanometer system comprising a movable coil type of galvanometerhaving a movable coil with end terminals, a pair of movable contactscarried by said coil, connections between saidcontacts and points. onsaid coil symmetrically disposed with respect to the electrical centerof the coil, a fixed contact cooperating with said two movable contacts,said galvanometer being of the zero center type, said galvanometerincluding conducting suspensions for said movable coil, a pair ofresistors connected in series with the outer terminals connected to thesuspension means so that the two resistors are connected across the coiland suspension means, a branch circuit between the iixed contact andcommon resistor terminal, said branch circuit including a source ofpotential and a load relay, said resistors having substantially the sameratio of Vresistance as one suspension means and the corresponding partof the coil up to the electrical center has to the remaining suspensionmeans and coil part, a source of current connected to the coil terminalsthrough the suspension means for actuating the movable coil toward acontact closing position, the polarity of the potential source beingsuch that when contact is established, current from said potentialsource passes through the galvanometer winding to generate additionalcontact closing force.

20. The galvanometer system according to claim 19, wherein saidgalvanometer coil has a section symmetrically disposed with respect tothe electrical coil center and extending to the movable contact taps,said section having substantially less resistivity than the remainder ofthe coil'.

STANLEY BREEN. MAXWELL A. KERR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,346,589 Lamb Apr. 11, 19442,357,524 Klepp Sept. 5, 1944 2,376,808 Ratz May 22, 1945-

